1. Field of Invention
The invention relates to a control apparatus of a hybrid vehicle equipped with two different drive units that supply driving force to a driving wheel side. More particularly, the invention relates to a control apparatus of a hybrid vehicle capable of preventing fluctuations in the output at the time of shifting carried out by a stepped transmission that is provided in one of the two drive units.
2. Description of Related Art
In recent years, various types of hybrid vehicles have been proposed in an attempt to improve fuel economy in view of adverse environmental effects and the like. Among such hybrid vehicles, a two-motor split type hybrid vehicle (see, e.g., Japanese Patent Application Laid-Open Publication No. 8-207601) has been proposed in which a rotating element of a planetary gear is connected to an output shaft of an internal combustion engine, and the other two rotating elements of the planetary gear are connected to a first electric motor and to a transfer shaft connected to driving wheels, and a second electric motor is connected to the transfer shaft.
In this hybrid vehicle, the first electric motor mainly receives a portion of the driving force of the engine and thereby generates electricity and simultaneously generates a reaction force on the rotating element of the planetary gear. The rest of the driving force of the engine is output to the transfer shaft via the rotating element on which the reaction force is generated by the first electric motor. Furthermore, a driving force of the second electric motor can be output to the transfer shaft. That is, the rotation of the engine can be controlled in a continuous or stepless fashion by controlling the first electric motor, so that the engine can be driven in a region of good efficiency. Additionally, the output to the driving wheels can be controlled via the second electric motor. Thus, on the basis of flexible control of the first electric motor and the second electric motor, the engine can be efficiently driven, and a driving force can be output to the driving wheels in response to a requested output from the driver.
In the above-described conventional hybrid vehicle (as found in Japanese Patent Application Laid-Open Publication No. 8-207601), the second electric motor is operated in association with the driving wheels. That is, the rotation of the second electric motor is increased as the vehicle speed increases. Generally, in a high rotation speed region of an electric motor, the output (torque) of the electric motor decreases in accordance with the rotation speed thereof. For example, when the vehicle is running in an intermediate-to-high speed region, the output of the second electric motor decreases as the rotation speed of the second electric motor becomes high. Further, in the case of vehicles that need great output power (e.g., vehicles having an engine displacement of 3000 cc, 4000 cc or the like), it is required that the capacity (output) of the second electric motor be large. In order to meet a request for great output in a high rotation speed region, a size increase in the second electric motor is needed, thus giving rise to a problem of impairment of easy installation into a vehicle.